1. Field of the Invention
The invention relates generally to modified chemical vapor deposition (MCVD) processes, and, more specifically, to MCVD processes used to manufacture fiber optic cable.
2. Description of Related Art
One existing process for forming optical fibers, described in U.S. pat. Nos. 4,334,903 and 4,217,027, is commonly referred to as modified chemical vapor deposition (MCVD). In this process, chemical vapors which will be used to form glass are introduced within a first end of a hollow, cylindrical glass starting tube. As the tube is rotated about its axis of symmetry, a localized heat source is moved back and forth along the length of the tube, thereby creating a "hot zone" within the tube. As the chemical vapors pass through this hot zone, they react to form particulate matter which then deposits along the wall of the tube in a direction downstream from the first end of the tube. Subsequent passages of the localized heat source consolidate the particulate matter into a substantially clear glass layer. After formation of one or more glass layers, the tube is collapsed into a solid structure, and fiber optic cable is drawn from the collapsed tube.
One of the most critical steps in determining the overall speed of the MCVD process is the rate at which deposition occurs. Research efforts directed towards increasing the deposition rate have identified the thermal gradient within the tube as a primary mechanism governing the deposition process. In general, when particulates are located within a thermal gradient, they migrate from the higher-temperature regions of the gradient to the lower-temperature regions. This movement of particulates is attributable to the effect of relatively energetic molecules in the higher-temperature regions of the tube colliding with particulates suspended in the gas stream, driving these particulates to the lower-temperature regions of the tube. As the moving heat source first passes near a given section of the tube, the area within the tube that is relatively close to the wall is heated to a higher temperature than the area close to the central axis of the tube. However, soon after the heat source moves away from this section of the tube, the area near the central axis of the tube remains relatively hot, whereas areas near the tube wall cool down to temperatures below that of the central axis. Downstream of the moving heat source, this applied thermal gradient causes particulates to migrate from the center of the tube, where they are first formed, to the tube wall upon which they are then deposited.
Various techniques have been developed for controlling the thermal gradient of the MCVD tube so as to speed up the deposition process. For example, U.S. Pat. No. 4,302,230 describes a technique of pouring water over the substrate upon which deposition occurs in order to thermophoretically enhance the deposition process. However, this technique is disadvantageous in that it subjects the tube to severe thermal shock and possible cracking. Moreover, the structural integrity of the deposited particulate matter may be severely compromised. What is needed is an improved technique for enhancing the speed at which MCVD operations may be performed.